Why Lily Allen got arrested by police?

A photo of Lily Allen was uploaded to Allen's Instagram account and Twitter, in that photo she was kneeling on the ground and handcuffing by a police officer, who is standing behind him. According to an Australian Federal Police (AFP) spokesman, she asked cops to place handcuffs on her, just for a photograph, it means that she was not getting arrested. The photo has been deleted from her Twitter account and Instagram account, but you can see that photo in many other websites.